AM630 behaves as a protean ligand at the human cannabinoid CB2 receptor

Background and purpose: We have investigated how pre-incubating hCB(2) CHO cells with the CB(2) receptor antagonists/inverse agonists, AM630 and SR144528, affects how these and other ligands target hCB(2) receptors in these cells or their membranes.

Experimental approach: We tested the ability of AM630, SR144528 and of the CB(1) /CB(2) receptor agonists, CP55940 and R-(+)-WIN55212, to modulate forskolin-stimulated cAMP production in hCB(2) CHO cells or [(35) S]-GTPγS binding to membranes prepared from these cells, or to displace [(3) H]-CP55940 from whole cells and membranes. Assays were also performed with the CB(2) receptor partial agonist, Δ(9) -tetrahydrocannabivarin. Some cells were pre-incubated with AM630 or SR144528 and then washed extensively.

Key results: AM630 behaved as a low-potency neutral competitive antagonist in AM630-pre-incubated cells, a low-potency agonist in SR144528-pre-incubated cells, and a much higher-potency inverse agonist/antagonist in vehicle-pre-incubated cells. AM630 pre-incubation (i) reduced the inverse efficacy of SR144528 without abolishing it; (ii) increased the efficacy of Δ(9) -tetrahydrocannabivarin; and (iii) did not affect the potency with which AM630 displaced [(3) H]-CP55940 from whole cells or its inverse agonist potency and efficacy in the [(35) S]-GTPγS membrane assay.

Conclusions and implications: These results suggest that AM630 is a protean ligand that can target a constitutively active form of the hCB(2) receptor (R*) with low affinity to produce agonism or neutral antagonism and a constitutively inactive form of this receptor (R) with much higher affinity to produce inverse agonism, and that the constitutive activity of whole cells is decreased less by pre-incubation with AM630 than with the higher-efficacy inverse agonist, SR144528.

Linked articles: This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.